Complex grammatical structures have been assumed to be best learned implicitly (Krashen, 1982, 1994; Reber, 1989). However, research to date has failed to support this view, instead finding that explicit training has overarching beneficial effects. The present study attempted to elucidate this issue by examining how type and token frequencies in incidental learning input and individual differences in the learner's working memory (WM) combine to affect the receptive and productive learning of a complex agreement pattern in a novel language. The findings indicated that type frequency significantly enhanced receptive knowledge acquisition even more than explicit instruction. Performance on the productive knowledge retrieval task was poor under all learning conditions but most accurate under the explicit learning condition. WM was not implicated in incidental learning, possibly indicating that all learners experience high cognitive demand imposed by the target structure regardless of variation in WM capacity.